Which people heading for amber stop? (4)
I believe the answer is:
whoa
'stop?' is the definition.
(command to stop a horse)
'which people heading for amber' is the wordplay.
'which people' becomes 'who' (I've seen this before).
'heading for' says to take the initial letters.
The first letter of 'amber' is 'a'.
'who'+'a'='WHOA'
(Other definitions for whoa that I've seen before include "Cry for horses to stop" , "Instruction to a horse to stop" , "Hold your horses!" , "The King of rock and roll" , "Said to stop a horse" .)